Histologic and scanning electron microscopic studies were made to determine the frequency of occurrence of endothelial cells on the surfaces of porcine valvular bioprostheses implanted in 43 patients. Endothelial cells were found to grow on the surfaces of bioprostheses as a function of time elapsed after implantation in the atrioventricular position; such cells were not found in bioprostheses that had been implanted in the aortic position. The growth of endothelial cells may serve to increase the structural stability of bioprosthetic cusps.